Methane -- four atoms of hydrogen bound to a carbon atom -- is the main component of natural gas on Earth. It is of interest to astrobiologists because organisms release much of Earth's methane as they digest nutrients. However, other purely geological processes, like oxidation of iron, also release methane. It will take future missions, like NASA's Mars Science Laboratory, to discover the origin of the Martian methane. You can read more about this topic at http://www.nasa.gov/mission_pages/mars/news/marsmethane.html